The present invention relates to a system for steering rear wheels of a motor vehicle.
A four-wheel steering system for steering rear wheels together with front wheels is known. Japanese Patent Publication No. 60-44185 discloses a rear wheels steering system where rear wheels are simultaneously steered in equiphase with the front wheels, thereby preventing response delay of the steering of the rear wheels.
Japanese Patent Laid-Open Nos. 58-164477 and 60-183264 disclose steering systems where steering of the rear wheels is delayed, in order to enhance the yawing, which is a movement of the vehicle to turn the direction thereof in accordance with the steering of the front wheels.
All of the above-mentioned systems are operated when a driver intentionally turns a steering wheel in order to go round a curve. Japanese Patent Laid-Open No. 62-187663 and EP No. 0243180 disclose steering systems, in which the swaying of a vehicle caused by a disturbing force such as wind is detected and the rear wheels are steered accordingly to steady the vehicle, although the steering wheel is at a neutral position for straight-ahead driving.
In the system, the sway of the vehicle caused by the disturbing force is detected as a cornering force exerted on the front wheels. However, a waveform of a detected signal dependent on the cornering force includes small fluctuations or discontinuities dependent on surface conditions of the road and frictions in the steering system. Thus, the steering of rear wheels cannot be smoothly controlled.
In order to smooth of the waveform of the signal, a low-pass filter is typically used to eliminate unnecessary components. However, the signal is attenuated by filtering. Accordingly, the steering angle of the rear wheels is extremely decreased. In addition, since the low-pass filter delays the signal in a high-frequency region, a high-frequency signal at quick steering is also delayed. Accordingly, the steering of the rear wheels is largely delayed when the steering wheel is quickly rotated.